FIG. 2 is a plan view showing an arrangement on a pixel unit of a solid-state imaging device. A light-to-electricity conversion part 15 comprising a photodiode, separation parts 16 for separating pixel regions from each other, and a CCD transfer part 17 for transferring light-to-electricity conversion signal generated in the light-to-electricity conversion part 15 are arranged as shown in FIG. 2. A solid-state imaging device includes a plurality of such unit pixels arranged in matrix on a chip. In addition, bonding pads for electrically connecting internal circuits of the chip with external circuits are provided on the chip.
Meanwhile, in order to improve the photosensitivity of a solid-state imaging device, a structure, in which a micro lens is provided on a substrate on which a light-to-electricity conversion element is formed, has been proposed. In this structure, light incident on regions other than on the light-to-electricity conversion element is collected by the micro lens, whereby the effective numerical aperture is improved.
FIG. 3 is a plan view schematically showing a solid-state imaging device having such a micro lens. In FIG. 3, a micro lens 22 is disposed on a light-to-electricity conversion element 21 spreading over and beyond the element 21. A bonding pad 23 is arranged on the periphery of the pixel region. This bonding pad 23 has a square or rectangular shape of a size equivalent to approximately ten pixels.
Although the space between adjacent micro lenses 22 is desired to be short to enhance light collection efficiency, a spacing of 0.5 to 1 micron is usually used considering production to tolerances.
Since the shape of micro lens 22 is optimized according to the shape of pixel, it is not restricted to the oval shape shown in FIG. 3, but may be circular. When the space between adjacent pixels in a longitudinal direction is short, a linear lens 22 as shown in FIG. 4 is employed for collecting light only from a horizontal direction. In this case, light collection efficiency of the micro lens 22 is lower than that of the micro lens shown in FIG. 3, but the production process is simplified. Also in FIG. 4, the space between adjacent micro lenses 22 is about 1 micron.
FIGS. 5(a) to 5(e) are cross-sectional views of process steps for producing the prior art micro lens. In these figures, reference numeral 1 designates a semiconductor substrate. Photodiodes 2 ar disposed in the surface region of substrate 1. Polysilicon gates 3 are disposed on the substrate 1 between the photodiodes 2. A bonding pad 6 is disposed at the periphery of the pixel region in which the photodiodes 2 and the polysilicon gates 3 are formed. An insulating layer 4 is disposed on the photodiodes 2, polysilicon gates 3 and substrate 1. Light shielding films 5 are disposed on the insulating film 4 opposing to the polysilicon gates 3. A base layer 7 of the solid-state imaging device is constituted by the substrate 1, photodiodes 2, polysilicon gates 3, insulating layer 4, light shielding films 5 and bonding pad 6. A flattening resin film 18 is disposed on the base layer 7.
A description is given of the production process.
First of all, a base layer 7 of a solid-state imaging device having light receiving parts 2, charge transfer parts or the like is prepared. Then, as shown in FIG. 5(a), a transparent flattening resin 18, such as thermally hardened resin or ultraviolet light hardened resin, is spin coated on the base layer 7 to a thickness of about 4 to 6 microns. This resin film 18 fills uneven parts of the base layer 7 that adversely affect the shape of micro lens. In addition, the resin film 18 is necessary for lengthening the optical path so that the micro lens may collect light efficiently.
Next shown in FIG. 5(b), a portion of the flattening resin film 18 on the bonding pad 6 is opened using photolithography. Thereafter, as shown in FIG. 5(c), a resin film 19 comprising a transparent material having thermal plasticity is formed on the entire surface to a thickness of about 2 to 3 microns by spin coating or the like. Then, the thermoplastic resin film 19 is partially etched away leaving portions thereof opposing the photodiodes 2, resulting in thermoplastic resin films 19' shown in FIG. 5(d). Finally, the thermoplastic resin films 19' thus patterned are heated up to the softening temperature or higher and deformed into a hemi-spherical shape, resulting in micro lenses 19" shown in FIG. 5(e). In the solid-state imaging device thus produced, since light incident on the light shielding films 5 can be collected by the micro lenses 19", the effective numerical aperture is significantly improved, whereby the photosensitivity of the solid-state imaging device is significantly improved.
The prior art method for producing micro lens of solid-state imaging device has the following drawbacks.
First of all, since the thermoplastic resin film serving as the micro lens must be formed of a material transparent to visible light, a material absorbing visible light cannot be used. More specifically, when a material having no photosensitivity is used for the thermoplastic resin film 19, a pattern of photoresist or the like must be formed on the thermoplastic resin film 19 and this pattern must be transferred to the thermoplastic resin film 19 using etching or the like. In this case, since it is difficult to detect the end point of the etching, it is necessary to select a flattening resin film 18 having higher anti-etching property than that of the thermoplastic resin film 19. However, such a flattening resin film 18 makes forming the opening for the bonding pad par difficult.
Meanwhile, it may be thought that a photoresist having sensitive to visible light may be used for the thermoplastic resin film 19 and after performing exposure, development and patterning directly, light irradiates the resin film 19 so that the resin film 19, i.e., micro lens, does not absorb anymore light. However, it is impossible to make the resin film 19 completely transparent. In addition, an extra resist is needed for opening the bonding pad part of the flattening resin film 18 and, therefore, deposition and patterning of the thermoplastic resin film must be carried out after opening the bonding pad part. However, since the difference at the opening is very large, such as 6 to 7 microns, when the thermoplastic resin film 19 is deposited thereon, non-uniformities extend radially from the opening part and influence on the shape of micro lens, resulting in non-uniformity sensitivities which appear on a screen at the time of imaging.
Furthermore, there is an effective method in which a photosensitive material having no visible light absorption, for example, a material which is sensitive to ultraviolet light or far-ultraviolet light, is used as the thermoplastic resin film and exposure and development are directly performed. However, in this method, it is necessary to perform the exposure using an i-line or excimer laser stepper (a reduction-type projection printing apparatus with i line or excimer laser as its light source) with considering the precision of the patterning and superposition on the base layer. This method is difficult to implement because of the cost of the device, producibility, and the like in the current state of semiconductor processing art.